Based on published data, the difference between the two methods for calculating the regional ventilation in the lower 50% ventilation volume is expected to have a standard deviation of about 0.1. Therefore, the 95% confidence interval for the difference in the regional ventilation between SPECT-CT and 4D-CT pre-treatment would have a width of no more than 0.14 if 10 patients are enrolled and as small as 0.10 when 20 patients are registered.